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Sensors and Actuators B 192 (2014) 648– 657

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locate /snb

oltammetric monitoring of Cd (II) by nano-TiO2 modified carbonaste electrode sensitized using 1,2-bis-[o-aminophenyl thio] ethanes a new ion receptor

oleyman Ramezania,∗, Massoud Ghobadib, Babak Nemati Bidehc

Faculty of Chemistry, Kharazmi (Tarbiat Moallem) University, Tehran, IranDepartment of Chemistry, Faculty of Science, Ilam University, Ilam, IranFaculty of Chemistry, Sharif University of Technology, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 27 August 2013eceived in revised form 6 November 2013ccepted 12 November 2013vailable online 20 November 2013

eywords:arbon paste electrode

a b s t r a c t

The objective of the work ahead is presentation of a new carbon paste electrode (CPE) modified by TiO2

nanoparticles (TiO2NPs) and 1,2-bis-[o-aminophenyl thio] ethane (APTE) ligand as a selective cationreceptor for determination of the Cd (II) ions using differential pulse anodic stripping voltammetry(DPASV). The electrode shows an excellent tendency to Cd (II) ions in presence of some interferingspecies. Under the optimum conditions, a linear calibration curve was obtained in the concentrationrange of 2.9 nM to 4.6 �M with a correlation coefficient of 0.9969 in the anodic potential of 0.54 (V vs.Ag/AgCl). The metal detection limit was 2.0 nM after 10 min preconcentration (S/N = 3). The sensor shows

iO2NPsifferential pulse voltammetry (DPV)admium (II)

high applicability in solutions with the optimal pH of 9.0 adjusted by 0.1 M phosphate buffer as supportingelectrolyte and open circuit accumulation optimum time of 10 min. The stability of the modified elec-trode was studied in the period of 2 months week by week. The results showed that for a series of severaldeterminations of Cd (II) at 25 �M relative standard deviations (RSDs) are below 3.4% for modified CPEs.Eventually, the electrode was successfully used for determination of Cd (II) in tap water, Livergol herbalmedicine and garden soil aqueous samples.

. Introduction

Undoubtedly, cadmium is one of the environmental countlessazards, and it’s exposures to environment comes primarily from

ossil fuel combustion, phosphate fertilizers, natural sources, ironnd steel production, cement production and related activities,onferrous metals production, and municipal solid waste incinera-ion [1]. Cadmium is one of six substances banned by the Europeannion’s Restriction on Hazardous Substances (RoHS) directive,hich bans certain hazardous substances in electrical and elec-

ronic equipment but allows for certain exemptions and exclusionsrom the scope of the law [2]. Bread, root crops, and vegetables alsoontribute to the cadmium in modern populations [3]. Long-termxposure to cadmium in contaminated food and water has resultedn a few instances of general population toxicity, and researches arengoing regarding the estrogen mimicry that may induce breast

ancer [3]. Concisely, for all of the mentioned reasons, the cadmiumII) like many chemical elements is extremely harmful to humannd other living organism health.

∗ Corresponding author. Tel.: +98 25 38745629.E-mail address: Soleyman.ramezani@yahoo.com (S. Ramezani).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.11.033

© 2013 Elsevier B.V. All rights reserved.

So, it is worthy to expand an accurate, rapid and sensitivemethod to monitor Cd (II) concentration in the environmental sam-ples. For this purpose, many instrumental methods such as atomicabsorption spectrometry (AAS) [4], inductively coupled plasmamass spectrometry (ICP-MS) [5], and atomic fluorescence spec-trometry (AFS) [6] have been used. Although these techniqueshave good selectivity and high sensitivity, they are somewhattime-consuming, expensive and not convenient for the in situmeasurement. On the contrary, the electrochemical techniques fordetermination of trace metals have attracted increasing interestsdue to their high sensitivity, rapid performance, portability, lowcost and the simple instrumentation. Because of the toxicity of mer-cury and the feasibility of formation of intermetallic compoundsin amalgams, numerous new electrode materials and methods aredesired to replace it. At present, several mercury-free electrodes,such as bismuth film electrodes [7], carbon nanotube modifiedelectrodes [8], nano-porous pseudo carbon paste electrodes (nano-PPCPEs) [9,10], acetylene black electrode [11], lead film electrode[12], boron-doped diamond electrode [13], nano-TiO2/chitosan

combination film [14] and carbon paste electrodes [15] are devel-oped for sensitive electrochemical determination of diverse metalsion in various aqueous and non-aqueous media. Among them,chemically modified carbon paste electrodes (CMCPEs) have seized

S. Ramezani et al. / Sensors and Actu

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the highly pure and uniform anatase crystals are obtained. Aftercalcination at 400 and 600 ◦C, most of the particles in the sampleshave diameters in the range of 10–50 nm. The presence of a certainamount of H2SO4 well expands the anatase phase and inhibits

NH2 H2N

Fig. 1. Chemical structure of the APTE.

pecial position and popularity among the electrochemists becausef their unique properties such as low cost, Availability of starteraterials, low background current, non-toxicity, simple usability,

igh conductivity, renewability, high atmospheric stability, andost importantly unparalleled adaptability with various chemicalodifiers.Nowadays, metal and metal oxides NPs are extensively used

o modify the electrochemical sensors and biosensors in varioushemical and physical methods. The properties of these materialsre quite different from those of their bulk counterparts, due toheir extremely small size, special structure and large surface-to-ulk ratio [16]. Among them, TiO2NPs is the promising material asemiconductor having high photochemical stability and low cost.

ell-dispersed Titania NPs with very fine sizes are promising inany applications such as pigments, adsorbents and catalytic sup-

orts. Hence, there is a considerable interest in using TiO2NPs as modifier since they have high surface area, optical transparency,ood biocompatibility and relatively good conductivity [17]. As aesult, in continuation of modification, the TiO2NPs was preparednd used in a physical method to amplify some essential charac-eristics of the introduced electrode.

Another option for improvement of CMCPEs sensitivity is appli-ation of the novel functionalized organic materials which arenown as ion receptor agents. It is evident that, the outstanding rolef the ion receptor compounds is in the preconcentration stage ofhe SADPV. These compounds could have a selective formation ofhe reversible organometallic complex with the target ions. Thishenomenon is not too surprising due to the presence of somelectron-donor functional groups which have high affinity to thelkali, alkaline earth and transition metal ions. Hereto, this strategyruly improves the preconcentration of the target ion on the elec-rode surface that impact extremely the modified sensor sensitivitynd selectivity [18].

In the study ahead, the synthetic compound of APTE (Fig. 1)as used as an appropriate mediator to improve selectivity and

ensitivity of the prepared CPE for stripping determination ofhe cadmium (II) in the aqueous media. The preliminary studiesere performed to evaluate the sensitivity of the APTE partic-lar compound toward certain ions by cyclic voltammetry (CV).he obtained results showed that the APTE have the most affinityoward Cd (II) ion among other heavy metal ions. Nevertheless, ithows partial tendency to Hg (II) and Cu (II) ions and can be ignoredue to its low quantity. Eventually, the Cd (II) ion concentration wasuccessfully detected in some real samples using standard additionethod through the SADPV three electrode systems.

. Materials and methods

.1. Apparatus

All voltammetric measurements were made by a conventionalhree-electrode system using a Potentiostat (Palm Sens, EN 50081-) and the proposed sensor (CMCPE) as a working electrode in

onjunction with an Ag/AgCl reference and a Pt wire counterlectrodes (Azar electrode, Iran). A Metrohm-827 pH/mV meterSwitzerland) was used for all pH measurements. Some solutionsere homogenized by ultrasonic homogenizer (Bandelin UW 3200,

ators B 192 (2014) 648– 657 649

Germany) whenever it was necessary. UV–vis absorption spectrumwas recorded with a UV–vis spectrometer (Perkin-Elmer modelLambda 25, USA). All measurements were carried out at the roomtemperature.

2.2. Reagent and materials

Analytical reagent grade chemicals and double distilled (DD)water were used to prepare all aqueous solutions at 25 ◦C. Car-bon graphite powder and paraffin oil were purchased from theFluka (USA). The titanium chloride (TiCl4, 99.90%) and H2SO4 (98%)were obtained from the Merck (Germany). The cadmium nitrate,and nitrate salts of other metals (all from Merck) were of thehighest purity available and used without any further purificationto prepare all of the metallic ions stock solutions. Synthesis andpurification of the APTE was performed in the Polymer and Macro-molecules Research Lab. of the Chemistry Department of KharazmiUniversity (Tehran, Iran) according to the reference [19] and usedwithout any manipulation. Buffer solutions were prepared fromHNO3, H2SO4, NaOH, H3PO4 and its salts for pH range of 3.00–12.00adjusted by pH/mV meter in order to diminish any errors. All otherchemicals used, were of the analytical reagent grade (from Merck,Aldrich or Fluka). All glassware used in the experiments werewashed with freshly prepared aqua regia and rinsed thoroughlyin tap water first and then DD water before using.

2.3. Preparation

2.3.1. Preparation of the TiO2NPsNano-crystalline anatase form of the titanium NPs were

prepared according to a special technique that is presented inreference [20]. Accordingly, TiCl4 and H2SO4 were used as themain starting materials. Fig. 2 shows the UV–vis diffuse reflectancespectra of the Titania samples calcinated at 400 and 600 ◦C. Thisabsorbance rate decreases and shifts to slightly higher wave-lengths as temperature of the powder increases. It clearly showsthat Titania has strong electronic absorption in the UV region.UV–vis technique was successfully used to confirm the formationof the TiO2NPs. Consequently, by using this particular method

Fig. 2. UV–vis spectra of the TiO2NPs calcinated at 400 and 600 ◦C.

650 S. Ramezani et al. / Sensors and Actuators B 192 (2014) 648– 657

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Fig. 3. XRD spectrum o

he anatase/rutile transformation even at 600 ◦C that give highonductivity to the TiO2NPs due to its uniform crystalline form. Inresence of the excess amount of the sulfuric acid, Raman spectrand electron diffraction confirmed that the nanoparticles are com-osed of anatase TiO2. NPs, no other Titania phases, such as Rutiler Brookite, were detected. (More details are in reference [20]).n addition, for more confirmations, the X-ray diffraction (XRD)nalysis of the TiO2NPs was well obtained (Fig. 3). It can be claimedhat, The XRD analysis agrees fully with the previously reportedRD analysis of the TiO2 nanostructure. The X-ray patterns of

he nanoparticles obtained by the above mentioned methodhows anatase crystalline nature of the NPs relying on the reasonsnterpreted in the relevant previously reported references [21].ventually, SEM was used in order to investigate the morphologyf the TiO2NPs. As can be seen clearly in Fig. 4, the morphology ofhe particles is close to sphere. The most of the nanocrystals in theample exhibit diameters in the range of 10–50 nm.

.3.2. Preparation of unmodified and modified CPE

Bare CPEs were prepared by mixing 66 mg of graphite pow-

er (heated at 700 ◦C for 15 min) with 34 mg of paraffin oil with mortar and pestle. The modified CPE was prepared by the sameethod except that the graphite powder was mixed with a desired

Fig. 4. SEM images of TiO2 nanopa

ase TiO2 nanoparticles.

amount of chemical modifiers of APTE ligand and TiO2NPs. Then,the mixture was dissolved in appropriate amount of acetone as amore volatile and inert solvent, and the resulting solution was wellhomogenized by ultrasonic homogenizer for 5 min, and then, thesolution was left at room temperature to evaporate thoroughly theexcess solvent. Both unmodified and modified pastes were packedinto a polyethylene tube (2.5 mm diameter), the tip of which hadbeen cut off. Electrical contact to the paste was established viainserting a copper wire thorough flank. A fresh electrode surfaceof paste was cut out with a glass rod and the exposed-end polishedon a slick paper until the surface showed shiny appearance.

2.4. Analytical procedure

Voltammetric determination process consists of preconcentra-tion (accumulation), deposition (cathodic electrolysis), stripping(detection), and regeneration steps (if necessary). During precon-centration step, the CMCPE was immersed in a 50 mL cell containing30 mL of metal ion solution at open circuit conditions for a specified

period of time (i.e., normally 10 min, but varied from 1 to 30 minin the preconcentration time study). During preconcentration, the50 mL cell containing 30 mL of Cd (II) ion and 0.1 M phosphate buffer(pH = 9.0) as a supporting electrolyte was stirred at over 350 rpm

rticles calcinated at 600 ◦C.

d Actuators B 192 (2014) 648– 657 651

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Fig. 5. Cyclic voltammograms of the (a, b) Bare-CPE, (c) APTE-CPE, (d) APTE-

S. Ramezani et al. / Sensors an

sing a magnetic stirring bar. After the pre-concentration step, theolution is allowed to achieve quiescence for 30 s. Then, a negativeathodic potential (0.1 V vs. Ag/AgCl) was applied to the electrode inhe phosphate buffer solution for 50 s. The potential is then scannedn the positive direction (i.e., from 0.1 to 1.0 V) with the scan rate of00 mV s−1, when the potential is sufficiently positive (0.54 V); annodic current is generated due to the oxidation of metals. The oxi-ation peaks grows and then dies down. Eventually, the data wereecorded for more analysis.

.5. Voltammetric behavior of Cd (II)

As was already hinted, the most probable mechanism of the CdII) electrochemical behavior on the APTE-TiO2NPs-CPE surface isearly as follow. At first, in order to accumulate the cadmium iont the CPE surface via formation of the APTE-Cd (II) complex, therepared electrode was immersed in to the unknown cadmiumolution in open circuit conditions for appropriate period of timeoptimal time is 10 min). In the next step, the Cd (II) accumulatedas reduced at 0.1 V and the product was then oxidized in the strip-ing stage in the applied potential swept from 0.1 to 1.0 V. The stepsan be briefly described as follows:

Cd2+)(solution) + (APTE)(CPE) � (Cd2+-APTE)(CPE) + 2H+

accumulationstage, opencircuit, 10 min, pH = 9.0) (I)

Cd2+-APTE)(CPE) + 2e− → (Cd0 + APTE)(CPE)

(depositionstage,Eapplied = 0.1 V, 50 s) (II)

d0(CPE) � Cd2+

(solution) + 2e−

strippingstage,Eapplied = 0.1–1.0 V,Ep = 0.54 V) (III)

. Results

.1. Step by step voltammetric characterization of the CPEodification

Cyclic voltammetry technique was used to follow up each ofhe modifying additives effect on the electrochemical responsef the proposed electrode. The perceptible cyclic voltammogramsecorded in 0.1 M of phosphate buffer solution with pH 9.0 as sup-orting electrolyte in the absence and presence of 25 �M of Cd (II)

on at unmodified (bare) CPE, APTE modified CPE and TiO2NPs-APTEodified CPE at the scan rate of 100 mV s−1 and accumulation opti-al time of 10 min are shown in Fig. 5. As the figure suggests, the

are CPE show no response in absence of Cd (II) ion due to no sig-ificant peak current (curve a). The trifle current that can be seen

n this manner, is definitely belongs to the back ground current ofhe CPE. Also, the curve b was recorded by the same electrode (barePE) in the presence of 25 �M of the Cd (II) solution. In this manner,

t is clearly evident that the observed slight current is due to sponta-eous physical adsorption of cadmium ions on the CPE surface after0 min open circuit preconcentration. By adding a certain amountf the APTE ion receptor to the carbon paste composition (APTE-PE), there is a significant increase in the peak current from 1.18 to.13 �A pertain to the unmodified and APTE modified CPE respec-ively, which suggested that presence of the APTE in the carbonaste composition improves markedly the sensitivity of the CMCPEcurve c). This phenomenon certainly arises from high affinity of

he APTE ligand to form APTE-Cd (II) complex and subsequentlyo accumulate the Cd (II) ion on the CPE surface in the preconcen-ration stage. In Comparison with the bare-CPE and APTE-CPE, theighest peak current of Cd (II) is obtained at the TiO2NPs-APTE-CPE

TiO2NPs-CPE in 0.1 M of phosphate buffer solution with pH 9.0 at scan rate of100 mV s−1. Electrodes were exposed in 30 mL of blank (a), and 25 �M of Cd (II)solution (b–d).

(curve d), demonstrating the strong preconcentration of Cd (II) atthe TiO2NPs-APTE-CPE surface. It is worth noting that, the improve-ment in the sensitivity of the CMCPE can certainly be attributedto the multi-functional characteristics underlined in theTiO2NPs-APTE combination: the high chelating ability of N and S atoms ofAPTE to Cd (II), the strong anodic ability of APTE for Cd (II), andpromotion of the surface area to volume ratio of the carbon paste.On the basis of the above-mentioned factors, the adsorption effi-ciency of Cd (II) on the TiO2NPs-APTE-CPE surface is much higherthan that at the bare CPE and the APTE modified CPE. Therefore,the stripping peak current of Cd (II) at the ultimate modified CPEincrease significantly that is the main objective of this research.Therefore, all subsequent experiments in this study, is performedsuccessfully by the TiO2NPs-APTE-CPE.

3.2. Carbon paste components optimization process

So far, a known fact about CPE that has been proven is thehigh impressionability of its efficiency by the type and amount ofthe carbon paste ingredients. Thus, the influence of presence andamount of ion receptor (APTE), TiO2NPs and amount of bindingagent (paraffin oil), on the voltammetric response (peak current)of the electrodes over Cd2+ ion concentration were evaluated care-fully and the results were summarized in Table 1. As can be seen, inthe absence of APTE, the electrode has low response (low peak cur-rent) toward cadmium (II) ion (No.1). Also, by increasing amountof the APTE (%wt.) in the carbon paste composition, the calibra-tion curve slope increases, manifesting the impressive effect of theion receptor (APTE) in the accumulation quantity and consequentlyits effect on the peak current (No. 2, 3 and 4). Super abound scaleof the ionophor amount in the carbon paste composition (Nos.5), albeit, resulted in a little decrease in electrode response as aresult of decrease of GP/ligand weight percentage ratio, and in con-sequence decreases of paste homogeneities and of course in theconductivity of the carbon paste [22]. In a separate assay, influenceof the TiO2NPs on the electrochemical response of the modified

electrode was evaluated. The results revealed that, peak current inpresence of the TiO2NPs grows obviously (No. 6–9). It is probablydue to enhancement of the surface/volume ratio of carbon pastebulk by Nano-scale dimension of the titanium dioxide crystals, as

652 S. Ramezani et al. / Sensors and Actuators B 192 (2014) 648– 657

Table 1The paste compositions and the electrode characteristics of cadmium (II) sensor.

No. GP (mg) APTE (mg) TiO2Nps (mg) PO (mg) Ip,a (�A)a LR (M) LOD (M)

1 66 – – 34 1.18(±0.3) – –2 65 2 – 33 4.11(±0.2) – –3 64 3 – 33 6.50(±0.2) – –4 63 5 – 32 8.13(±0.4) 1.5 × 10−3 to 2.3 × 10−6 2.0 × 10−6

5 62 6 – 31 5.11(±0.1) 1.8 × 10−3 to 2.6 × 10−5 2.3 × 10−5

6 63 4 2 31 13.20(±0.3) 2.4 × 10−3 to 2.7 × 10−6 2.0 × 10−6

7 62 4 3 31 14.70(±0.1) 3.4 × 10−3 to 1.8 × 10−7 1.0 × 10−7

8 61 4 4 31 15.80(±0.3) 2.2 × 10−3 to 2.2 × 10−8 2.0 × 10−8

9 61 5 4 30 16.58(±0.3) 2.2 × 10−3 to 2.2 × 10−9 2.0 ×10−9

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a Values in parentheses are SDs based on three replicate analyses of Cd (II) 25 �M

ell as diminishing the Ohmic resistance of the carbon graphiteaste in consequence of the TiO2NPs high electrical conductivity23]. As the data in Table 1 suggest, at the end of the optimiza-ion process of the carbon paste ingredients weight percentageatio, the APTE-TiO2NPs-CPE with the highest performance (higheak current) and APTE/TiO2NPs/GP/PO optimal percentage ratiof 5/4/61/30 was found and used as a working electrode in voltam-etric quantitative assessment of Cd (II) ion aqueous solution

nd supplementary experiments (No. 9). In continuation of inor-inately increasing weight percentage ratio of the both modifiers,he observed electrode response comes down significantly (No. 10nd 11). This phenomenon absolutely depends on the impressiveecrease in the carbon paste uniformity.

.3. Electrolyte and pH-dependence of peak current

Since, the APTE-Cd (II) complex formation depends likely on thenvironment pH value (preconcentration step), it can be one of theffective variable parameters, which strongly influenced the peakurrent and shape of the stripping voltammograms. Therefore, atrst, the pH-dependance of the Cd (II)-APTE complex strippingeak current was investigated in the pH range of 3.0–12 usingyclic voltammetry in 0.1 M phosphate buffered solution with fixedonic strength and the results were shown in Fig. 6. As can be seen

vertly from the figure, by enhanced growing up of the pH from 3.0o 9.0, the peak current Continuously promote due to increasinghe complex formation of Cd (II) with APTE at the electrode surfaceith increasing pH and the completion of complex formation

ig. 6. Variation in solution pH in the wide range of 3.0–12 and its effect on theoltammetric response of the APTE-TiO2NPs-CPE to Cd (II) ion (applied scan rateas 100 mV s−1 in presence of phosphate buffer as supporting electrolyte).

14.50(±2.0) 2.2 × 10 to 2.5 × 10 2.3 × 1014.20(±2.0) 2.2 × 10−1 to 2.3 × 10−5 2.0 × 10−5

ion.

at higher pHs of supporting electrolyte. But, in the higher thanpH 9.0, immediately, the peak current value begins drastically todegrease. Unmistakably, This could be due to the formation ofmetal hydroxide complexes of Cd(OH)2 that is indubitably insol-uble, by providing of the Ksp required OH− value of the Cd(OH)2precipitation in the high concentration of OH− (Ksp = 7.2 × 10−15,pH = 9.0). In the one hand, these hydroxide complexes by pre-cipitating either on the inner wall of the electrolytic cell or onthe electrode surface in contact with the solution, performs asinhibitor and occupant of the active sites of CMCPE. In the otherhand, by forming relatively insoluble hydroxide precipitation, theidentifiable ions are kept away from access of the sensitive sitesof the electrode. Thereby, it causes a significant decrease in thequantity of solution phase electrode-detectable metal ions thatmay reach to the electrode surface, thereupon, causes a significantdecrease in the analytical parameter of peak current. Therefore,the pH 9.0 was selected as an optimum condition in order to obtainmaximum sensitivity in quantitative analytical measurements.

Furthermore, the effects of some common electrolytes, such asKCl, HCl, HNO3, Britton–Robinson, citrate, acetate, and phosphatebuffer solutions, as supporting media on the peak currents of Cd (II)were carefully compared. It was found that Cd (II) had the best elec-trochemical responses (high peak current and free from interferingpeaks) in phosphate buffer solution. Therefore, phosphate buffer(pH 9.0) was chosen as supporting electrolyte for voltammetricdetermination of the Cd (II) in the supplementary experiments.

3.4. Effect of preconcentration and deposition time

Looking for more analytical parameters effective on DPASVpeak current of Cd (II) ion at the surface of proposed CMCPE,the effect of preconcentration and deposition time were evalu-ated and the results were shown in Fig. 7a. It was found that thesurface concentration of the cadmium (II) species increase withincreasing preconcentration time until the saturation surface con-centration is reached. As can be seen in Fig. 7a, the peak current(�A) versus preconcentration time (min) increased linearly untilthe surface became saturated, where the peak current is almostindependent of preconcentration time. It was found that 600 s isan appropriate period of time for a successful and complete pre-concentration. Moreover, the deposition time effect on the anodicstripping peak current (Ip,a) was studied in the range of 10–100 s.It was found that, Increasing the time required for deposition ofcadmium ions adsorbed on the electrode interface immersed inthe phosphate buffered solution in pH of 9.0 containing 25 �M ofCd (II) ion causes to increase discernibly the observed peak cur-rent in constant applied potential of 0.1 V vs. Ag/AgCl (Fig. 7b). As

it can be seen clearly in Fig. 7b, after elapsing 50 s the observedpeak current is independent from the deposition time represent-ing a restricted amount of the adsorbed Cd (II) ion on the CPEsurface.

S. Ramezani et al. / Sensors and Actu

Fig. 7. Effect of varying (a) accumulation time and (b) deposition time on the DPASVp0

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period of not-using time, a fresh surface of the APTE-TiO NPs mod-

eak current of 2.5 × 10−5 mol L−1 Cadmium ion in presence of phosphate buffer.1 M; scan rate 100 mV s−1, pH 9.0.

.5. Effect of scan rate

In order to investigate the anodic peak current (Ip,a) proportionith potential scan rate, the applied potential was swept from 10

o 100 mV s−1 in 25 �M of Cd (II) phosphate buffered solution withdjusted pH around 9.0 (Fig. 8a). The resulting data were evalu-ted as an Ip,a (�A) versus v and (v)1/2 (mV s−1) variation curveFig. 8b and c). As can be seen overtly, peak current for anodic oxi-ation of Cd (II) (Ip,a) at the surface of modified electrode shows

linear variation with potential sweep rate (v) in the range of0–100 mV s−1 manifesting an adsorption-like behavior of corre-ponding mechanism. Statistical data for this regression analysisIp,a = b� + a) shows a slope of 0.1244 and R2 = 0.9991. The resultsllustrate that the electrocatalytic oxidation of Cd (0) at the sur-ace of APTE-TiO2NPs modified electrode may be proceed via thereating of an adsorbed adduct of Cd (II) ion to form a reversibled (II)-APTE complex by N and S electron donor atoms of APTEation receptor that well coordinate with Cd (II) ion center (see Sec-ion 2.5). In support of this claim, the low correlation coefficientsR2 = 0.97) of the Ip vs. (v)1/2 plot subscribes that the electrodeurface processes do not have a diffusion-controlled behavior. Fur-hermore, by enhancing the scan rate, the peak potential (Ep) shifted

o more positive values, approving the kinetic limitation in thenterfacial dynamic process that occur at the CPE/solution inter-ace (Fig. 8 d). For evaluation of the kinetic parameters, there

ators B 192 (2014) 648– 657 653

is a useful device that called Tafel plot. In general, there is ananodic branch with slope, (1 − �)F/2.3RT and a cathodic branchwith slope −�F/23RT. For the Cd (II)/Cd(0) system in phosphatebuffered solution, There is a linear relation between Ep,a and log vin the range of 10–100 mV s−1 (Fig. 8e). The linear regression equa-tion is [Ep,a = 0.0105 log v (±0.0052) + 1.0551(±0.005), R2 = 0.9143].The Tafel slope (b) should be linear with a slope of 0.0105 using[Ep,a = (b/2) log v + Constant] equation. Therefore, it was found that,the Tafel slope is 0.021 V by applying n = 2 in the 0.1 M phosphatebuffered solution at the ATPE-TiO2NPs modified electrode/solutioninterface. Hereby, the mass transfer coefficient (˛), was found

= 0.24 in according to above noted anodic branch slope. Accord-ingly, both anodic and cathodic processes contribute significantlyto the currents measured in the over potential (�) range where masstransfer effects are not important [24,25].

4. Discussion

4.1. Distinguishing figure of merits of the electrode

4.1.1. Linear range and limit of detectionEmphatically, two of the most important and most widely appli-

cable figure of merits of the ion selective electrodes (ISEs), arelinear range (LR) and limit of detection (LOD). Hence, these twodistinguishing analytical characteristics of the modified electrodeare needed to be evaluated after preparing the modified electrodeusing DPASV. Hereby, the Cd (II) ion stripping Ip,a (�A) versus E (V vs.Ag/AgCl) curves at various concentrations in the 0.1 M phosphatebuffer at pH 9.0 was shown in Fig. 9. Notably, The DPV parame-ters were 50 mV pulse amplitude, pulse width 50 ms, and a scanrate 100 mV s−1. Also, the Ip,a (�A) versus – [Cd (II)] (�mol L−1) plot(Fig. 9) suggests a wide dynamic LR of 2.9 nM to 4.6 �M with aLOD of 2.0 nM (S/N = 3) and correlation coefficient of R2 = 0.9969.An electrochemical sensor with such unparalleled high sensitivitycan be achieved only in presence of some good modifier additivesthat possess high performance modifying effect.

4.1.2. Lifetime and reproducibilityUnmistakably, stability and reproducibility of the chemical sen-

sors, intensively depends more than anything on the chemicalnature of the electrode components especially on the modifieradditives. It is noteworthy, in this particular case, the CPE ingre-dients including APTE and TiO2NPs have adequate chemical andatmospheric stability for construction a proper chemical sensor.The operational reproducibility of the modified electrodes was sur-veyed by monitoring the DPV in 25 �M Cd2+ solution via 5 diverseAPTE-TiO2NPs modified sensors built in the same conditions aspossible. The obtained results (Cd (II) stripping peak currents) ofthe 5 electrodes had the lowest possible relative standard devia-tion (RSD = 4.0%) representing high reproducibility of the proposedsensor.

Also, the lifetime of the APTE-TiO2NPs modified electrode wasstudied within two months by periodically (week by week) recali-brating the sensor applicable in electrochemical stripping analysisof 25 �M of cadmium (II) standard solution. No significant changein the electrode performance (LOD) was observed after two monthsstorage (8 replicate experiments, RSD = 5.0%). Certainly, the secretof long lifetime for the modified electrode lies in the high atmo-spheric and chemical stability of the CPE ingredients. It should benoted that, for long time storage, the electrode must be kept in DDwater and in cool dark place. To reuse the electrode after a long

2ified electrode could be produced by scraping out the old surfaceby a smooth graph or weighing paper. Of course, to achieve an ade-quate stable and repeatable response, the electrode surface should

654 S. Ramezani et al. / Sensors and Actuators B 192 (2014) 648– 657

F lution1 . v and

bo

4

iaToecptd

ig. 8. (a) Cyclic voltammograms of the APTE-TiO2NP-MCPE in phosphate buffer so00 mV s−1 respectively, and variation of the (b) Ip,a vs. v, (c) Ip,a vs. (v)1/2, (d) Ep,a vs

e perfectly smooth and free of any scratches that much dependsn the skill of the examiner.

.2. Interference study

As the name implies, selectivity is one of the most critical andmportant characteristics of the ISEs which often can be excellencegent of an electrode to the other with the same analytical aim.hereby, performance of the proposed electrode for determinationf the cadmium in presence of some interfering ions was carefullyvaluated. For this purpose, the influence of other metal ions, which

an compete for complexation at binding sites on the APTE com-ound embedded throughout of the carbon paste, were tested usinghe same procedure as that optimized for cadmium quantitativeetermination and the results were summarized in Table 2. This

(pH 9.0) containing 25 �M of Cd (II) at various a to f scan rates of 10, 20, 40, 60, 80, (e) Ep,a vs. log v (all experimental conditions are exactly same as Fig. 3).

table surveys the results obtained by studying the selected inter-ference ions at concentrations of 100 times as large as that of Cd(II). The interaction of some possibly more interfering species suchas; Pb (II), Ag (I), Hg (II), Cu (II), Zn (II), Ni (II), Co (II), NO3

−, F−,SCN− ions and some biological species with the ASDPV peak currentwas investigated by adding increasing amounts of these interferingspecies to a solution containing 25 �M of Cd (II). The data presentedin this table clearly shows that, the peak current intensity of Cd(II) was not dramatically affected by lower concentrations of thesecations. Nonetheless, at high concentrations, 100-fold excess overCd (II) in the analyte solution, the interfering ions reduced the elec-

trode response by competing with cadmium (II) ions for the bindingsites. These results are in agreement with our previous research,which includes amplification of the response of ion selective elec-trode to a given target ion by presence of a selective ion receptor

S. Ramezani et al. / Sensors and Actu

Fig. 9. DPAS voltammograms of APTE-TiO2NPs-MCPE in the various Cd (II) con-centrations in the range of 2.9 nM to 4.6 �M (Conditions: 0.1 M phosphate buffersolution as a supporting electrolyte, pH adjusted at 9.0, DPV parameters were 50 mVpulse amplitude, pulse width 50 ms, and a scan rate of 100 mV s−1).

Table 2Changes of the cadmium (II) (25 �M) peak current, affected by some interferenceions with concentration of 2.5 × 10−3 M.

Interference ion Peak current changes (%)

Hg2+ −7.0Cu2+ −5.0Ag+ −4.5Pb2+ −6.0Zn2+ −3.8Ni2+ −4.0Co2+ −3.3SCN− −4.5NO3

− −1.5F− −2.2Cysteine −3.1Fructose −2.4

(tCtrAwT

adaptable with the results of FAAS method (data are not reported).

TC

Urea −3.2Glucose −4.8

an organic compound) [26,27]. It is noteworthy that, in generalhe response and selectivity mechanism of the some ion selectiveMCPEs that use some synthetic organic compounds as ion recep-or additives, obey somewhat from the hard-soft acid base (HSAB)ule of Pearson. It is therefore expected that, the active sites on the

PTE compound containing sulfur and nitrogen soft atoms are moreilling to some soft ions such as Cd (II). Nevertheless, according to

able 2 some species such as Hg (II), Cu (II), Ag (I) and Pb (II) ions

able 3omparing the performance of the proposed electrode with some cadmium (II) electrode

Modified electrode Method LR (m

Lead film electrode SWV 2.0 ×SbNPs BDEa LSV 4.5 ×Nano-TiO2/chitosan composite film DPV 1.0 ×Zirconium phosphated amorphous silica DPV 2.7 ×Antimony film CPE ASV 4.5 ×Kaolin modified Platinum electrode SWV 9.0 ×N-p-chlorophenyl-cinnamohydro-xamic acid modified CPE DPV 4.0 ×Zeolite-modified electrode CV 5.0 ×Thiol-functionalized mesoporous film and ILs-GCE LSV 2.9 ×TiO2NPs-APTE-CPE DPV 2.2 ×a Antimony nanoparticle modified boron-doped diamond electrode. NR, not reported.

ators B 192 (2014) 648– 657 655

interfere partially with Cd (II). This phenomenon is quite consistentwith the HSAB rule of Pearson.

After this, another factor affects the selectivity of the electrodeis various electroreduction potentials of the APTE-Ion complexes.Certainly, in the potential range of the 0–1 V vs. Ag/AgCl only theAPTE-Cd (II) complex can be reduced. Therefore, spontaneously,other interfering species cannot interfere with Cd (II) ion.

As Table 2 suggests discernibly, The Hg (II) interfere more thanother ions shown in the table. Peak current of Cd (II) decreasedrelatively in the presence of Hg (II) because it can be reducing atthe surface of the electrode and form a mercury film. The interfer-ence can be eliminated using appropriate masking agent. The effectof other interfering metal ions arises from partial interaction withAPTE that are in competition with Cd (II) ion.

4.3. Comparative study

In Table 3, some of the proposed electrode figures of merits werecompared with number of previously reported Cd (II) voltammetricsensors. As can be seen, the proposed electrode in some respectsis better than other Cd (II) sensors. As shown in the table, thereare a few reports [12,14] that have lower LOD than this study. But,one of the greatest prominences of the TiO2NPs-APTE-CPE to otherelectrodes shown in Table 3 is the wide linear range that certainlygoes back to the type of the modifiers. Apart from this, the presentedsensor has better performance in selectivity rather than some of thepreviously reported electrodes [29,31,32].

4.4. Analytical performance

At the end, it was necessary to evaluate the electrode perfor-mance for quantitative determination of the cadmium ion in realenvironments. To do so, the proposed electrode was utilized asa sensitive and selective working electrode for determination ofCd (II) in different environmental (tap water, herbal medicine andgarden soil) samples. Since differential pulse voltammetry (DPV)has a much higher current sensitivity and a better resolution thancyclic voltammetry, it was used to estimate concentration of Cd (II)using the standard addition method (SAM) and the results wereshown in Table 4. The pharmaceutical and soil samples were care-fully prepared according to the literature [34] and [35] respectively,and the water samples were used directly and there were no needfor any pretreatments. All experiments conditions used are exactlysame as above mentioned optimal conditions. As it is obvious, thedetermination of cadmium in each sample has been performedsatisfactorily accurate and the obtained results were completely

This is demonstrating the fact that, the proposed Cd (II)-CMCPE canact very well as a working electrode in voltammetric determinationof Cd (II) ion without any fault or defect (Table 4).

s reported previously.

ol L−1) LOD (mol L−1) Interfering ion Ref.

10−9 to 1.0 × 10−6 6.0 × 10−10 No [12] 10−7 to 4.5 × 10−6 3.4 × 10−7 No [13]

10−9 to 1.0 × 10−6 2.0 × 10−10 No [14] 10−8 to 1.2 × 10−5 2.6 × 10−8 No [28] 10−8 to 4.5 × 10−7 7.1 × 10−9 NR [29]

10−8 to 8.3 × 10−6 5.4 × 10−9 No [30] 10−8 to 1.6 × 10−7 9.8 × 10−9 Cu2+, Pb2+ [31] 10−8 to 1.0 × 10−6 8.9 × 10−9 Fe3+ [32]

10−8 to 8.7 × 10−4 1.0 × 10−9 No [33] 10−9 to 2.0 × 10−3 2.0 × 10−9 No This work

656 S. Ramezani et al. / Sensors and Actuators B 192 (2014) 648– 657

Table 4Quantitative determination of cadmium (II) in some real samples.

Sample Cd2+ (mg L−1)a Compatibility (%)

Added (SAM) Found (MCPE)b

Tap water 010.0

Nd c

10.1(±0.2)–101.0

Livergol herbal medicines 010.0

Nd10.8(±0.3) (mg g−1)

–108.0

Garden soil 010.0

Nd10.2(±0.2) (mg g−1)

–102.0

a Values in parentheses are SDs based on three replicate analyses.b Optimal conditions: supporting electrolyte; phosphate buffer in adjusted pH of 9.0, scan rate; 100 mV s−1, pulse amplitude; 50 mV, pulse width; 50 ms, preconcentration

a

5

doscTaCooirsawcC

A

LUa

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nd deposition time; 10 min and 50 s respectively.c Not detected.

. Conclusions

According to the obtained experimental results, it is clearly evi-ent that the APTE synthetic compound and the TiO2NPs have wellbviated the urgent need to a suitable CPE modifier. These sub-tances based on its unparalleled characteristics make impressivehange in the electrochemical response of the modified electrode.he presented APTE-TiO2NPs-CPE displayed well-defined, sharpnd highly reproducible stripping peaks for low concentrations ofd (II) with a minimum background current. The best results werebtained using the combination of APTE/TiO2NPs/GP/PO in theptimal weight percentage ratio of 5/4/61/30 (mg). Such electrode,llustrated a good LOD of 2.0 nM over a wide linear concentrationange of 2.9 nM to 4.6 �M. Meanwhile, the CMCPE presents a goodelectivity over Cd2+ ion contaminated by some of the inorganicnd biological interfering species. Thereupon, a novel Cd (II) CPEas successfully designed and manufactured with some prominent

haracteristics compared with previously reported electrochemicald (II) sensors.

cknowledgement

Our research group is very grateful to General Chemistry (I)aboratory staff and the headman of Kharazmi (Tarbiat Moallem)niversity faculty of chemistry for their sincere cooperation in theccomplishment of this research.

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iographies

oleyman Ramezani currently is a research assistant of Dr. Ahmad Ali Pourbabaein the Department of biotechnology, university collage of agriculture and natural

ators B 192 (2014) 648– 657 657

resources, University of Tehran, Karaj, Iran. He received a BS degree in 2009 fromArak University, Arak, Iran and MS degree in 2012 from Kharazmi (Tarbiat Moallem)University, Tehran, Iran. His current research interests involve chemically modifiedsensors and biosensors.

Massoud Ghobadi currently is doing Ph.D. studies on synthesis and characteriza-tion of phosphine ligands in Ilam University, Ilam, Iran. He received a pure chemistryBS degree in 2009 from Arak University, Arak, Iran, and inorganic chemistry MSdegree in 2012 from Ilam University, Ilam, Iran.

Babak Nemati Bideh currently is Ph.D. student of Zanjan University, Zanjan, Iran.He received a pure chemistry BS degree in 2009 from Arak University, Arak,Iran, and inorganic chemistry MS degree in 2012 from Sharif University of tech-nology, Tehran, Iran. His current research interests involve dye sensitized solarcells.